1. Field of the Invention
This invention relates to a dispersible colorant and a method of producing the same as well as to an aqueous ink using the same for ink jet recording, an ink tank, an ink jet recording apparatus, an ink jet recording method and an ink jet recorded image.
2. Related Background Art
The ink jet system is a system for recording images, characters or the like by forcing fine ink droplets to fly from a nozzle on the basis of any of various known actuating principles and causing them to arrive at a recording medium (e.g., a sheet of paper). The ink jet system is characterized by the easiness of realizing high speed and low noise multicolor printing, the versatility of recorded patterns and the disuse of development and fixing processes, which rapidly have made the ink jet system very popular for a variety of applications. Furthermore, the development of the full color or multicolor ink jet recording system in recent years has made them comparable with multicolor printing by conventional plate making and color photography in terms of forming high quality multicolor images. Particularly, the color ink jet recording system is finding various applications in the field of full color image recording because it can produce less expensive printed matters if compared with ordinary multicolor printing and color photography when the number of copies is small.
Many improvements have been made to ink jet recording apparatus and ink jet recording methods to meet the demand for a higher recording speed, a higher level of image precision and perfect full color images. Requirements to be met by ink for ink jet recording apparatus include (1) it provides uniform images of high resolution and high optical density that are free from blurring and fogging, (2) it shows a high ejection responsiveness and a high ejection stability and does not give rise to any clogging due to ink drying at a tip of an ejection nozzle, (3) it fixes well on sheets of paper, (4) it provides images of good fastness and (5) it has good storage stability for a long period of time. Particularly, high speed printing that has been achieved in recent years requires the use of ink that dries and fixes quickly to provide high quality images even if it is used on plain paper such as copying paper.
Intensive efforts have been and being paid for developing a colorant by using a pigment that is excellent in waterfastness and weatherability of image and water-insoluble so as to make them operate as ink for ink jet recording. As a matter of fact, such colorants are being used in the field of large size printers. Additionally, researches have been and being made for inks that contain pigments for the purpose of ink jet recording by printers for office use and personal use. Sheets of plain paper are dominantly being used as recording medium in such applications and the requirements that are being posed on such inks are rigorous than ever particularly in terms of image fixability, rubfastness, quality of printed characters, image density, waterfastness and other characteristics probably because such inks are desirably equally usable for glossy paper and other special paper.
When using a water-insoluble colorant, a pigment in particular, for an aqueous ink to be used for ink jet recording, it is firstly required that the colorant can be dispersed stably into water. Generally, when a water-insoluble colorant is used for an aqueous ink to be used for ink jet recording, a surfactant or a polymeric dispersing agent (also referred to as “dispersing resin”) is used to disperse/stabilize the colorant.
For instance, Japanese Patent Application Laid-Open No. H10-120955 discloses an ink for ink jet recording having a colorant dispersed and stabilized by means of a dispersing resin. However, the disclosed ink is accompanied by a problem that because the dispersing resin itself is highly affinitive with water and is therefore highly soluble into water, the dispersing resin can easily leave the surface of the colorant and hence the colorant cannot remain in a sufficiently stabilized state for a long period of time. Additionally, when such ink impacts against the surface of plain paper, the highly hydrophilic dispersing resin will function as a penetrating agent and penetrate deep into the plain paper with the colorant, thus making it impossible to obtain a sufficient image density. Further, as a result of using an ink containing a polymer component such as a highly water-soluble dispersing resin for ink jet recording, there is a case where the ink adheres to a nozzle of the ink jet recording apparatus both at the outside and in the inside thereof. Then, as a result, both the size and the moving direction of the ejected ink droplets become unstable to consequently give rise to disturbance of image, thereby posing a problem in obtaining a highly precise image. Additionally, when the content of a water-soluble dispersing resin is increased in order to improve the rubfastness of an image formed on plain paper or glossy paper, the viscosity of the ink will increase to make it very difficult to maintain the ejection stability of the ink from the ink jet recording apparatus.
Japanese Patent Application Laid-Open No. 2001-081372 discloses an ink for ink jet recording having a colorant dispersed therein by means of polyoxyethylenealkylether sulfate, which is a surfactant. When a colorant is dispersed by means of a surfactant, the surfactant that is firmly adsorbed to the colorant may function also as a penetrating agent. Therefore, as the ejected ink impacts against the surface of recording paper, the colorant will penetrate deeply into recording paper, thereby making it hardly possible to obtain a high print density. Additionally, a colorant that is dispersed by means of a surfactant is free from an agent such as a resin that provides a bonding action on the surface of recording paper and hence the image formed by using such a colorant may pose a problem that the rubfastness and marker resistance (tolerance to marker pen) of a formed image can not be obtained sufficiently.
Known methods of stably dispersing a colorant without using a dispersing resin or a surfactant include techniques for chemically modifying the surface of a water-insoluble colorant. For example, Japanese Patent Application Laid-Open No. H10-195360 discloses an aqueous pigment ink using, as a colorant, a self-dispersing type carbon black formed by bonding a hydrophilic group to the surface of carbon black directly or through another group. Such a surface chemically modified pigment is referred to as a self-dispersing pigment and shows good ink jet ejection stability because it does not need any water-soluble resin or the like. However, according to the consideration of the present inventors, such a colorant does not contain any resin and therefore has a poor adhesion to recording paper, so that the rubfastness and marker resistance thereof need to be improved.
Independently of the above-described dispersing technique, Japanese Patent Application Laid-Open No. H08-183920 discloses a method of dispersing a coloring agent in an organic solvent, subsequently enclosing the coloring agent with a self-dispersing resin by phase conversion emulsification, followed by dispersion in water and removal of a solvent. The resulting pigment is a so-called microcapsule pigment and, also in this case, in order to attain sufficient dispersion stability, it is necessary to increase the hydrophilicity of the coating resin. However, consideration by the present inventors has revealed that while the dispersibility is improved by increasing the hydrophilicity, the coating resin may sometimes be detached from the pigment surface to thereby increase the viscosity of the ink, so that it becomes difficult to strike a balance between the ejection stability and the dispersion stability.
Other techniques for enclosing a coloring agent with a resin include those using aqueous precipitation polymerization as described in “Colorants”, 68 [9], 1995, p. 535-541, Japanese Patent Application Laid-Open Nos. H05-222109, H07-010911, H09-100303 and 2003-034770. Although the above listed “Colorants”, 68 [9], 1995, p. 535-541 and Japanese Patent Application Laid-Open No. H05-222109 describe encapsulation of a pigment by aqueous precipitation polymerization and completing of a pigment and a resin, the described particle size is large and hence the dispersion stability is insufficient for use in aqueous ink jet recording.
Japanese Patent Application Laid-Open Nos. H07-010911 and H09-100303 disclose a method of producing dispersed particles of pigment having a particle size suitable for ink jet recording by dispersing a colorant in advance by means of a reactive surfactant and polymerizing the reactive surfactant and other monomers to precipitate a resin, thereby coating or modifying the surface of the colorant. Furthermore, Japanese Patent Application Laid-Open No. 2003-034770 discloses a technique of dispersing a colorant by means of a surfactant that satisfies certain special requirements and subsequently conducting aqueous precipitation polymerization. With these techniques, the resulting colorant that is modified by the resin is essentially dispersed and stabilized due to the existence of a surfactant or a water-soluble polymeric dispersing agent in the ink. Therefore, as described above, it is very difficult to strike a balance between the ejection stability that is suited to an ink jet recording apparatus and the sufficient printing quality of a recorded image particularly from the viewpoint of the print density.
As described above, there have been proposed methods of treating a water-insoluble colorant, particularly the surface of various pigments for dispersing by means of a resin or surfactant, methods that employ chemical modification and methods that employ microencapsulation. However, each of the proposed methods is accompanied by problems to be dissolved in terms of image density, ejection stability, rubfastness, marker resistance, dispersion stability and so on. Particularly, they need to be improved with regard to establishment of sufficient image density and rubfastness in plain paper recording, and compatibility thereof with sufficient ejection stability.
Furthermore, WO 02/066564 discloses an ink that contains, as coloring agent, both a pigment having a water-dispersibility-donative group incorporated into a surface thereof and a pigment having a surface enclosed with a water-dispersible polymer. Although the disclosed ink has been developed aiming at simultaneous improvement of both color developability on plain paper and rubfastness on exclusive paper, it merely has a simple composition in which the both pigments having the respective characteristics are mixedly contained and is therefore not so satisfactory as to allow the two pigments, after being arrived at the paper, to remedy each other's deficient property. In short, the disclosed ink that contains the two kinds of pigments having the respective characteristics is not satisfactory in terms of both color developability and rubfastness and still has a room for improvement.
Finally, Japanese Patent Application Laid-Open Nos. H09-152342 and 2001-152060 disclose an ink realized by using a microencapsulated pigment that is formed by coating a water-insoluble colorant such as carbon black with an organic polymer and regulating the relationship between the colorant and the coating resin. More specifically, the color density, the preciseness, the color rendering property, the transparency and the color developability are improved by regulating a pigment content and a resin coating ratio. However, the disclosed methods employ the conventional technique of microencapsulation using phase conversion emulsification and hence are characterized in that the resin coat which covers the surface of a colorant is not uniform.